Low energy electrons focused by the image charge interaction in carbon nanotubes

Electrons with energy in the range of a few eV are strongly affected by the interaction with the polarization charges they induce on a surface. This report shows how this effect is relevant for the data analysis of inverse photoemission spectroscopy (IPS) from carbon nanotube (CNT) arrays. IPS from CNTs exhibit two main resonances, located around 2.5eV and 12.5eV above the Fermi level. The intensity of the first resonance is dependent on the average tube diameter and the second one has a distinctive spectral shape, which is related to the graphitization level of the CNT external walls. In order to analyze the origin of these resonances, a phenomenological reconstruction of an IPS spectrum from a CNT collection was performed. This reconstruction successfully reproduces the spectral shape of the 12.5eV resonance. However, the intensity is lower than the actual measurements in the initial energy range of the spectrum. The analysis of these results suggests that the additional intensity required to reproduce the experimental data, has its origin in an electronic focusing mechanism, induced by the CNT image charge potential. This effect is significant for low energy electrons and small diameter tubes.